1. Field of the Invention
The invention relates to an electro-dynamic transducer for non-destructive testing of workpieces by means of ultra-sonics. It consists of a magnet with a magnet yoke with external pole shoe surrounding a conically-shaped inner pole shoe. This inner pole shoe is tapered preferably towards the workpiece, and equipped in the region of its free face opposite to the workpiece with exciter and receiver coils, which in turn are provided with a cover at least on the side facing the workpiece.
2. Description of Prior Art
A corresponding electro-dynamic transducer can be seen from GB-A-20 06 433. The magnet, which--as usual--takes the form of an electro-magnet, comprises two coaxially arranged poles the geometries of the sides facing the workpiece to be tested corresponding to the geometry of the workpiece. Arranged on the front face of the inner pole shoe, which also consists of two mutually arranged coaxially arranged sections, is an exciter coil and a receiver coil. The workpiece side of this coil is covered by a layer of ceramic material, whereas the magnet pole side is covered by an insulating layer. If the pole shoes are appropriately designed then, amongst other things, feed-back from the workpiece under test and associated generation of eddy currents in the pole shoes can have an adverse influence on the resolution of the measurement signal. Neither is it possible to lower an appropriate electro-dynamic transducer head in the area of the exciter and receiver coil directly onto the workpiece in order to carry out measurements with a rotating electro-magnet and/or rotating workpiece. This is because the surface of the exciter and receiver coil would be immediately destroyed by the abrasive effect of the workpiece. The cover made of ceramic material is not highly impact-resistant however.
Described in the DE-A-31 23 935 is an electro-dynamic transducer, which also contains an inner pole shoe surrounded by an external pole shoe, whereby the inner pole shoe has on its face an exciter and receiver coil in addition to radially orientated slots into which transformer stampings can be placed, if required. The aim of this feature is to eliminate feed-back from the tested workpiece to the pole shoes, thus enabling high resolution of the measurement values. An appropriate electro-dynamic transducer head cannot, however, be lowered onto the tested workpiece--particularly a tube--and also moved, since this would result in immediate destruction of the measurement system.
In the European Patent Registration 00 45 412 and the British Letters Patent 14 25 201 electro-dynamic transducers are described in which the exciter and receiver coils are arranged on the inner pole shoe of a magnet with heating insulation. During measurement a clearance is left between the face of the transducer and the workpiece to be tested. For this purpose an air cushion is generated between the workpiece and the transducer in accordance with GB-PS 14 25 520. Described in DE-OS 28 45 579 is a transducer in which the surfaces of an inner pole shoe and the surrounding outer pole shoes are concave, and matched to the exterior shape of the workpiece being tested. The curved surfaces of the pole shoes are also spaced from the surface of the workpiece.